582  Preparing  Pharmaceutic  Chemicals.  {^fS^il)™' 
lent  quantity  of  pure  precipitated  barium  carbonate  is  added,  and 
the  mixture  boiled  for  3-6  hours  when  the  reaction :  Fel2  + 
BaC03  ^Z^FeC03  -f-  Bal2  takes  place.  The  equation  is  a  reversible 
one,  but  proceeds  from  left  to  right  in  the  beginning,  much  more 
rapidly  than  in  the  opposite  direction,  as  in  a  state  of  equilibrium 
only  a  very  small  amount  of  iron  iodide  is  present.  According 
to  the  law  of  mass  action  the  speed  of  a  reaction  is  propor- 
tional to  the  concentration  of  the  reacting  substance.  If  there- 
fore one  of  the  products  of  a  reversible  reaction  be  removed,  the 
reaction  will  go  on  in  the  direction  to  form  more  of  that  product. 
Hence,  when  ammonia,  added  to  the  filtered  solution  produces 
only  a  slight  precipitate,  which  after  prolonged  boiling  does  not 
diminish,  the  equilibrium  is  reached,  and  the  iron  carbonate  should 
be  removed  by  nitration.  The  slight  amount  of  barium  carbonate 
in  solution  will  usually  be  sufficient  to  precipitate  the  iron  from 
the  filtered  solution,  which,  consequently,  after  cooling  and  standing 
for  a  few  hours,  becomes  turbid,  and  after  renewed  filtration  will 
be  found  perfectly  free  from  every  trace  of  iron.  Should  this  not 
be  the  case,  a  further  boiling  with  a  little  barium  carbonate  will 
throw  out  all  the  iron,  and  a  solution  of  pure  barium  iodide  will 
result. 
This  solution  is  now  diluted  to  a  definite  volume  and  its  strength 
accurately  determined,  which  can  be  done  by  precipitating  the 
barium  with  sulphuric  acid  and  weighing  the  sulphate.  The  author 
prefers  however  to  determine  the  iodine  by  the  Volhard's  method 
(the  direct  titration  with  potassium  chromate  as  indicator  can  not 
be  used  on  account  of  the  insolubility  of  barium  chromate),  which 
is  both  rapid  and  very  accurate.  When  the  amount  of  barium 
iodide  is  known  such  a  calculated  quantity  of  a  dilute  (10-20  per 
cent.)  exactly  standardized  sulphuric-acid  solution  is  added  as  to 
cause  an  exact  precipitation  of  the  barium.  The  barium  sulphate 
settles  quickly,  and  if  the  strength  of  the  solutions  were  accurately 
determined  the  resulting  hydriodic  acid  solution  gives  only  a  slight 
precipitate  either  with  sulphuric  acid  or  barium  chloride  test  solution. 
As  barium  as  an  impurity  is  more  objectionable  than  sulphuric  acid, 
the  solution  should  be  fixed  by  the  addition  of  small  amounts  of 
sulphuric  acid,  so  as  to  give  just  a  very  slight  cloud  with  barium 
chloride  test  solution,  when  it  is  filtered  or  decanted  and  evaporated 
down  to  the  desired  concentration.  The  acid,  which  is  decomposed 
to  a  slight  extent,  is  decolorized  by  boiling  it  10-15  minutes  with  a 
